1. Field of the Invention
This invention relates to a spring brake actuator. More specifically, the invention relates to a spring brake actuator with a guide and bearing for an actuator rod.
2. State of the Prior Art
A brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is controlled by a brake actuator. Conventional air brake actuators have both a service brake actuator and an emergency brake actuator. The service brake actuator applies the brakes under normal driving conditions by the application of compressed air. The emergency brake actuator includes a strong compression spring which applies the brake when air is released. This actuator is often referred to as the spring brake actuator. Typically, the spring brake actuator is disposed in tandem with a service brake actuator.
A chamber in the spring brake is normally pressurized with air. One wall of the chamber is moveable and typically comprises either a diaphragm or a piston. It, in turn, acts against the compression spring so that pressure in the chamber compresses the spring.
In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator chamber, the brake will be mechanically activated by the force of the strong compression spring acting on a spring brake actuator rod which, in turn, acts upon the service brake actuator or directly to apply the brakes. Thus, the spring brake actuator serves both as a parking brake and an emergency brake. In some applications the spring brake actuator is designed to operate solely as a parking brake, as for instance, when the service brake actuator is hydraulically operated.
U.S. Pat. No. 5,105,727 to Bowyer, issued Apr. 21, 1992, discloses a known spring brake assembly which includes both a spring brake actuator and a service brake actuator. The service brake actuator includes an air chamber partially defined by a flexible diaphragm acting against a service brake push rod and a return spring to assure proper release of the brake when air is exhausted from the air chamber. The spring brake actuator also includes an air chamber comprising a spring brake housing and a spring brake actuator rod disposed integrally with a spring brake pressure plate and secured to the spring brake diaphragm. The spring brake diaphragm is provided with a centrally disposed aperture and engages the actuator rod and pressure plate to form an airtight seal. A strong power spring acts between an end of the spring brake housing and the pressure plate, and tends to urge the actuator rod out of the air chamber to engage the diaphragm and push rod of the service brake and to cause the brake to be applied. During normal operation, the spring force is counteracted by air pressure in the spring brake air chamber which acts against the diaphragm to compress the power spring, retracting the actuator rod. When system air pressure is lost, or manually released, the power spring extends to apply the brakes.
The actuator rod is hollow and provided with a central bore. The proximal end (nearest the spring brake housing) of the actuator rod is partially closed by an inwardly directed annular flange on a pressure plate. A brake releasing caging bolt extends into the central bore from outside of the spring brake housing and has a head for engaging the annular flange. The bolt is threaded and extends out of the actuator rod, through an aperture in the spring brake housing, through a threaded nut, and terminates in a wrench nut.
The compression spring does not always extend evenly in an axial direction so that the pressure plate and the actuator rod attached to it can wobble or become cocked. Consequently, the caging bolt head may abrade the interior wall of the actuator rod as it moves when the emergency brake is applied or released.